Some critics fear the social network is working with governments to silence any criticism of the refugee crisis.

His censorship comments have now gone viral, provoking a debate about whether it is right to squash unpopular or potentially offensive views.

Mark Zuckerberg visits the United Nations headquarters

Zuckerberg was overheard after leaving his microphone on during a conversation about the refugee crisis with Angela Merkel.

"Are you working on this," the German chancellor asked him, according to Bloomberg.

"Yeah," he replied.

The super-rich tech boss also said "we need to do some work on the issue".

"We are committed to working closely with the German government on this important issue,” said Debbie Frost, a Facebook spokeswoman.

"We think the best solutions to dealing with people who make racist and xenophobic comments can be found when service providers, government and civil society all work together to address this common challenge.”

Mark Zuckerberg in the first live video at Facebook HQ (Image: Facebook)

Many observers hailed the move as a blow against racism and xenophobia.

But others feared the campaign was an assault on free speech.

On Twitter, prominent right-wingers said the censorship would make George Orwell's 1984 "become a reality".